FIG. 1 is a side-sectional view illustrating part of a conventional linear compressor, and FIG. 2 is a side-sectional view illustrating a piston of the conventional linear compressor.
Referring to FIG. 1, in the conventional linear compressor, one end of a cylinder 2 is fixedly supported by a main body frame 3 in a hermetic space inside a shell (not shown), and one end of a piston 4 is inserted into the cylinder 3. A compression space P is formed between the cylinder 2 and the piston 4. The piston 4 is connected to a linear motor 10 and reciprocated in the axial direction, for supplying refrigerants into the compression space P and discharging the refrigerants.
Here, the compression space P for compressing the refrigerants is formed between one end inner portion of the cylinder 2 and the piston 4. A communication hole 4b′ is formed at one end of the piston 4 in the axial direction for supplying the refrigerants into the compression space P. A thin suction valve 6 for opening and closing the communication hole 4b′ is bolt-fastened to one end of the piston 4. A discharge valve assembly 8 for discharging the refrigerants compressed in the compression space P is installed at one end of the cylinder 2.
In the discharge valve assembly 8, a discharge valve 8a is disposed to block one end of the cylinder 2, and a discharge cap 8b is fixed to one end of the cylinder 2, for temporarily storing the compressed refrigerants before externally discharging the refrigerants. The discharge valve 8a is elastically supported in the axial direction inside the discharge cap 8b by spiral discharge valve springs 8c. 
The linear motor 10 includes a ring-shaped inner stator 12 fixed to the outer circumference of the cylinder 2 and formed by laminating a plurality of laminations in the circumferential direction, a ring-shaped outer stator 14 disposed outside the inner stator 12 with a predetermined gap, and formed by laminating a plurality of laminations in the circumferential direction outside a coil winding body formed by winding a coil in the circumferential direction, and a permanent magnet 16 disposed in a space between the inner stator 12 and the outer stator 14, and linearly reciprocated by mutual electromagnetic force of the inner stator 12 and the outer stator 14.
One end of the inner stator 12 is supported by the main body frame 3, and the other end thereof is fixed to the outer circumference of the cylinder 2 by a fixing ring (not shown). One end of the outer stator 14 is supported by the main body frame 3, and the other end thereof is supported by a special motor cover 22. The motor cover 22 is bolt-fastened to the main body frame 3. The permanent magnet 16 is connected to the other end of the piston 4 through a special connection member 30.
Accordingly, when current is applied to the outer stator 14, the permanent magnet 16 is linearly reciprocated by mutual electromagnetic force of the inner stator 12 and the outer stator 14, and the piston 4 is linearly reciprocated inside the cylinder 2. As the internal pressure of the compression space P is varied, the suction valve 6 and the discharge valve 8a are opened and closed, for sucking, compressing and discharging the refrigerants.
The piston 4 applied to the conventional linear compressor will now be explained with reference to FIG. 2. The piston 4 is manufactured by casting, and comprised of a cylindrical piston main body 4a formed long in the axial direction, a compression unit 4b for blocking one end of the piston main body 4a, and a connection unit 4c extended from the other end of the piston main body 4a to the radial direction.
A guide hole 4a′ in which the refrigerants flow is formed in the axial direction in the piston main body 4a, at least one communication hole 4b′ for guiding the refrigerants flowing along the guide hole 4a′ to the compression space P is formed on the compression unit 4b, and at least one fastening hole 4c′ to which the connection member 30 is bolt-fastened is formed on the connection unit 4c, for connecting the piston 4 to the permanent magnet 16 of the linear motor 10.
Normally, low cost steel is cast into the piston 4 in a larger size than a real size. Mechanical processing such as turning and polishing is carried out on the outer circumference of the piston 4, for transforming the piston 4 to the real size. In addition, an oil circulation groove for circulating the oil, and a friction unit rubbing against the inner circumference of the cylinder 2 may be formed on the piston 4. As the piston 4 is manufactured by casting, although the piston 4 rubs against the inner portion of the cylinder 2, the friction intensity can be maintained.
However, since steel is cast into the piston 4 of the conventional linear compressor, defects frequently occur. The added processing such as turning and polishing increases the processing cost. As various holes are formed by cutting, burrs are generated to seriously reduce operation efficiency.
FIG. 3 is a perspective view illustrating the cylinder of the conventional linear compressor.
Low cost steel is cast into the cylinder 2 in a larger size than a real size. Mechanical processing such as turning and polishing is carried out on the inner and outer circumferences of the cylinder 2, for transforming the cylinder 2 to the real size. Therefore, blowhole defects frequently occur during the casting, thereby increasing a fraction defective. After the outer circumference of the cylinder 2 is mechanically processed, aluminum is die-cast into the frame 3, and the frame 3 is fixed to the outer circumference of the cylinder 2. Here, the cylinder 2 is too much mechanically processed before the die-cast and fixation of the frame 3, which increases the processing cost and decreases operation efficiency.